CO2 Tail Gas Purification And Recovery Process in Oil Plants

In recent years, under the national energy conservation and emission reduction policy requirements, petroleum refining enterprises and other enterprises have generally been equipped with carbon dioxide recovery devices, and the production capacity of carbon dioxide recovery devices has increased rapidly. However, due to the impact of public health events, the commissioning time of new carbon dioxide devices has been generally delayed. The downstream carbon dioxide market involves many industries, and their development conditions are different. The demand for carbon dioxide in the future is also different. The growth points of downstream industries are mainly concentrated in dry ice, food preservation, chemicals, oil field injection and oil recovery, etc. Carbon dioxide is mainly used in the chemical industry to supplement carbon for downstream products. The main application industries include the production of dimethyl carbonate, glacial acetic acid and other products. It is expected that the demand for carbon dioxide in industrial production will increase in sync with industrial production. Research on high-value chemicals from carbon dioxide has made continuous breakthroughs, such as carbon dioxide to methanol, olefins, aromatics, gasoline, carbon dioxide to formic acid, carbon dioxide and methane reforming to synthesis gas, carbon dioxide to degradable plastics, etc., which are expected to be mass-produced in the next few years. In addition, the application of dry ice as a cutting fluid in industrial production has also made significant progress.

At present, the main methods for industrial recycling and utilization of carbon dioxide include physical absorption and chemical absorption, adsorption separation, membrane separation, catalytic oxidation, low-temperature distillation and other processes. These technologies all use the differences between the physical or chemical properties of carbon dioxide and its associated components to separate or purify them. Each method has its advantages and disadvantages, and the appropriate method should be selected based on the recovery conditions.

Keywords: carbon dioxide; recovery technology; gas purification; liquefaction and purification

The absorption method is a widely used absorption method at home and abroad, mainly including physical absorption and chemical absorption. In the solution, when there is a high pressure in the solution, high pressure and low temperature absorption is usually used, while reduced pressure heating is used during analysis. The selected absorption material has good selectivity for carbon dioxide, high solubility, stable properties, and no corrosive effect. Good physical absorption materials include: polyethylene glycol, acrylate, ethanol, monoethanolamine dimethyl ether and methanol. The chemical absorption method refers to the reaction of carbon dioxide with certain absorption solvents to produce an intermediate compound, while other gases will not react with the solvent. The intermediate compound produced will be decomposed into a carbon dioxide and a solvent in another device. This method can continuously discharge carbon dioxide, and the absorption solvent is recycled, and finally realizes the separation of carbon dioxide from other mixed gases. The characteristics of carbon dioxide with good selectivity, stable performance, low toxicity, low corrosiveness, non-volatile, non-flammable, and no new pollutants have been studied. Commonly used are methyldiethanolamine compounds, monoethanolamine, etc.

The absorption method is generally suitable for treating gases with a carbon dioxide concentration of less than 20%. The advantages of this method are fast reaction speed, ability to effectively absorb large amounts of carbon dioxide, good separation effect, and the solvent required for the reaction can usually be recycled and reused. The disadvantages are that a lot of energy is required to heat the solvent, and the solvent itself may have a risk of contamination.

The adsorption separation method uses the interaction between the absorbent and carbon dioxide to achieve the separation of carbon dioxide. The adsorption method can be divided into three methods according to the working temperature and working pressure: variable temperature, variable pressure, and variable temperature and variable pressure. The absorber is used to absorb carbon dioxide under high temperature and high pressure conditions, and then decomposes it after cooling and reducing the pressure, and the carbon dioxide is separated by changing the temperature and pressure cycle. Traditional adsorption materials include molecular sieves, alumina and activated carbon. The variable temperature absorption technology is generally used for projects containing a small amount of impurities or difficult to remove, and is a commonly used technology in the current industry.

The adsorption separation method is generally suitable for treating gases with a carbon dioxide concentration of less than 80%. The advantage of this method is that the process is simple and impurities that are difficult to volatilize can be effectively removed by adsorption. The disadvantages are large investment, high energy consumption, and short service life of the adsorbent.

The membrane separation method uses the pressure difference of carbon dioxide as the driving force of the separation process to separate different gas permeabilities and diffusion coefficients. During the separation process, the mixed gas passes through the membrane material. Due to its molecular size and polarity, the carbon dioxide molecules can pass through the membrane material, while other gas molecules are blocked on the membrane surface, thereby achieving effective separation and recovery of carbon dioxide. Membrane materials are usually made of polymers, ceramics, metals and other materials, with different permeabilities and selectivities. Commonly used membrane materials include cellulose acetate membrane, polyethersulfone membrane, polypeptide membrane, polyimide membrane, polyphenylene oxide modified membrane, etc.

The membrane separation method is suitable for treating gases with clean gas sources and carbon dioxide concentrations below 80%. There is currently no large-scale industrial example. Its characteristics are small footprint, simple operation, low energy consumption, and low one-time investment. The disadvantage is that this method requires pre-treatment, filtration and dehydration processes, with a high purification rate, and impurities will affect the life cycle of the membrane.

The catalytic oxidation method mainly converts hydrocarbon-containing substances in the carbon dioxide raw gas into carbon dioxide and water. This method can effectively remove impurities in the raw gas, and the purification degree reaches 10 to 12 levels, but the process is complicated, and the energy consumption and cost are high.

The cryogenic distillation method mainly uses the difference in boiling points of the components of the raw gas to separate the raw gas through a distillation tower. One is liquefaction separation, which uses cryogenic technology to liquefy and separate carbon dioxide. The other is condensation separation, which solidifies and separates carbon dioxide at a lower temperature according to the different condensation temperatures. Cryogenic distillation recovers gases with a carbon dioxide concentration higher than 90%. The disadvantage is that the separation effect is not good, and low temperature can easily cause equipment and facilities to be blocked, so it is generally rarely used.

This combined process is to pressurize, dehydrogenate and dry the waste gas of the petroleum refining unit containing carbon dioxide, and gradually remove various light hydrocarbons, oxygen-containing organic matter, carbide and water components in carbon dioxide in a targeted manner, and then through liquefaction distillation and purification steps, it can meet the national food-grade product standards, and obtain high-purity food-grade liquid carbon dioxide and dry ice with a purity of more than 99.996%.

In recent years, the standard requirements for industrial-grade and food-grade carbon dioxide have become increasingly higher, and the use of carbon dioxide recovered by a single process purification method in industry and food processing has been greatly restricted. Taking the carbon dioxide tail gas produced by a 1 million t/a ethylene glycol unit as a raw material gas to produce food-grade liquid carbon dioxide and dry ice as an example, the process technology route selection is carried out.

The tail gas produced by the ethylene glycol unit is about 30 t/h, with a temperature of 60 ℃ and a pressure of 0.03 MPa. The content of each component in the tail gas is: carbon dioxide ≥ 80%, water: 17%, ethylene: 100×10-6 ~ 750×10-6, chloride ions: 1×10-6 ~ 3×10-6, and trace amounts of ethylene oxide. These impurities cannot be removed stably and effectively by absorption, adsorption, distillation and other methods. At present, the most effective method for removing C2 and above (including oxygen-containing organic matter) at home and abroad is catalytic oxidation. The combined process of catalytic oxidation and low-temperature distillation can be used to purify carbon dioxide. The annual operation of this process can reach 8,400 hours, and the raw gas processing capacity is 40% to 110%. The product indicators can fundamentally meet the food grade quality standards. The purity of carbon dioxide can reach 99.99%, the product quality is stable, and all indicators can be better than the "National Food Safety Standard Food Additive Carbon Dioxide" GB 1886.228-2016 standard or ISBT standard, and there is no wastewater or waste liquid discharge during the production process, no VOCs in the tail gas, and the tail gas emission indicators of the project meet the requirements of national and local emission standards.

The gas purification system includes a water cooler, an adsorption tank, a dehydrogenation preheater, a dehydrogenation oxidation tower, a dehydrogenation water cooler, a precooler, and a dryer.

The carbon dioxide exhaust gas from the ethylene glycol unit has a high temperature and contains saturated water. In order to improve the efficiency of the compressor, the raw gas needs to be cooled. After being cooled and separated by the water cooler, the raw gas is pressurized by the carbon dioxide gas compressor and enters the adsorption tank. The oil, water, chlorides and other toxic substances that may be carried are filtered out to protect the dehydrogenation catalyst. After being heated by the dehydrogenation preheater, it enters the dehydrogenation purification tower. Under the action of the active catalyst (precious metal catalyst) and a certain temperature, all hydrocarbons (including oxygen-containing organic matter) and other combustible components in the carbon dioxide gas react with oxygen to generate carbon dioxide and H2O. After the heat is recovered by the dehydrogenation preheater, it goes to the dryer to remove moisture. The total hydrocarbon content in the purified and dried gas is ≤48×10^-6, non-methane hydrocarbons ≤18×10^-6, and moisture ≤18×10^-6.

Main reaction equation:
C₂H₄+3O₂→2CO₂+2H₂O+Q
C_xH_y+O₂→xCO₂+y /2H₂O
C_xH_yO+O₂→xCO₂+y /2H₂O
All are catalytically oxidized to CO₂ and H₂O.

The drying system uses multifunctional molecular sieve adsorbents, and can choose grease adsorbents, chloride adsorbents, water absorbents, desiccants and desiccants, which are conducive to further purification of carbon dioxide gas. Adsorbent regeneration can adopt a more environmentally friendly fully closed cycle regeneration process, directly using the heat energy of catalytic dehydrogenation for regeneration, with low energy consumption and no exhaust gas emissions, ensuring a stable carbon dioxide recovery rate of the device, and only condensate is discharged during the regeneration process.

The liquefaction and purification system includes a residual cold recovery device, a liquefier, a purification tower, a reboiler, and a subcooler.

After drying, the carbon dioxide gas enters the residual cold recovery device and exchanges heat with the air discharged from the top of the purification tower. After recovering the cold, it enters the liquefier and is liquefied under the refrigeration of Freon. The outlet carbon dioxide liquid is controlled at about -16 ℃, and then enters the purification tower for distillation. The lower part of the purification tower is controlled by a reboiler to control the tower kettle temperature at -13 ~ -15 ℃, the tower top evaporation temperature at -30 ℃, and the pressure is controlled at about 2.2 MPa. The carbon dioxide evaporated with the non-condensable gas (O2, N2, etc.) is further cooled and recovered to improve the yield. Since the temperature of the liquid in the lower part of the purification tower is relatively high, in order to reduce storage losses, a subcooler can be set after the purification tower. The evaporation temperature of the subcooler is controlled at -30 ℃, so that the temperature of the carbon dioxide liquid is reduced to -25 ℃ for storage in a low-temperature spherical tank. The product is stored in carbon dioxide spherical tanks, part of which is transported by tank trucks and part of which is used as dry ice raw material.

The refrigeration process uses the evaporation characteristics of medium and low pressure Freon to cool and liquefy carbon dioxide gas at different temperatures, which can greatly improve the liquefaction efficiency of carbon dioxide gas.

Dry ice can be produced by reducing the pressure of liquid carbon dioxide to normal pressure, vaporizing part of the carbon dioxide into gas, and then cooling the other part of the liquid into snowflake-like solids, and then squeezing it into blocks or granules through a refrigerator. After packaging, it is placed in an insulated box and delivered to customers by car. The air released by dry ice is exchanged with the outlet gas of the dry ice compressor through the dry ice recovery cooler, and then enters the dry ice gas compressor for compression. After the compressed gas is heat exchanged, it is connected to the system liquefier inlet for recycling.

Waste gas: The drying bed and adsorption bed are regenerated regularly, and the non-condensable gas of the distillation tower is used as the regeneration gas source. The discharged regenerated tail gas meets the direct discharge standard and can be discharged directly. For the tail gas containing a small amount of ethylene and other substandard tail gas under abnormal working conditions, it can be incorporated into the waste gas and waste liquid incineration system for incineration treatment. Generally, the flue gas residence time is ≥2 s, and the combustion efficiency is ≥99.9%, which meets the standard discharge of waste gas under abnormal working conditions.

Wastewater: The sewage discharge volume of this process is 6 m³ /h, and the discharge volume is small, which can be directly used as circulating cooling water.

Solid waste: This process will only generate solid waste during the maintenance period, mainly adsorbent, molecular sieve and dehydrogenation catalyst; activated carbon is a hazardous waste, the molecular sieve is silicate, which is a general waste, and the dehydrogenation catalyst contains precious metals, which is a general fixed waste. After the above solid waste is generated, it can be sent to a qualified unit for disposal after being temporarily stored in the hazardous waste/general solid waste warehouse.

The liquid product carbon dioxide can be transported to the carbon dioxide spherical tank through pipelines. The inlet pipeline should be equipped with a cut-off valve and a carbon dioxide spherical tank level gauge interlock. In order to prevent the danger of the carbon dioxide spherical tank being full of liquid, a high-high interlock of the spherical tank level gauge is set to close the liquid carbon dioxide into the spherical tank pipeline cut-off valve; in order to prevent the danger of the carbon dioxide spherical tank being evacuated, a low-low interlock of the spherical tank level gauge is set to close the spherical tank outlet pipeline cut-off valve; the liquid carbon dioxide loading pipeline is equipped with a cut-off valve and a carbon dioxide spherical tank level gauge interlock, and the low-low liquid level interlock closes the loading pipeline cut-off valve; the liquid carbon dioxide to dry ice machine pipeline is equipped with a cut-off valve and a carbon dioxide spherical tank level gauge interlock, and the low-low liquid level interlock closes the dry ice machine pipeline cut-off valve; the filling pump in the loading process is equipped with a flow meter limit interlock to close the spherical tank outlet pipeline cut-off valve.

Online analytical instrument systems such as total hydrocarbon analyzer, trace sulfur analyzer, benzene analyzer, oxygen analyzer, and intelligent dew point meter are set up to sample and analyze the inlet and outlet of the carbon dioxide adsorption tank, the outlet of the dehydrogenation water cooler, the outlet of the dryer, the outlet of the purification tower vent, the outlet of the purification tower kettle, the vent main pipe, the outlet of the cryogenic spherical tank filling pipeline, the outlet of the cryogenic spherical tank dry ice removal pipeline, and the outlet of the cryogenic spherical tank vent. A combustible gas online detector is set after the raw gas adsorption tank, and online detection and analysis of oxygen content and hydrocarbon content are set at the outlet of the dehydrogenation oxidizer respectively. The components and characteristics of the raw gas at each processing stage are analyzed in real time, and each monitoring data signal is introduced into the DCS system for real-time monitoring, and its concentration is controlled to be less than 25% of the lowest value of the lower explosion limit of the most explosive component and the mixed gas. When the combustible gas LEL% in the inlet mixed gas exceeds 25% of the lower explosion limit, the system will interlock and cut off the dehydrogenation oxidizer feed valve to prevent explosive gas from entering the dehydrogenation oxidizer under abnormal conditions in advance, so as to ensure safe and reliable operation of the device.

The emergency pressure relief system considers the pressure circuit protected by the safety device (safety valve), and selects the largest one from the discharge conditions that can be caused by the failure of each key component as the pressure relief condition setting value of the safety valve. Pressure vessels and pipelines are equipped with safety valves in the event of overpressure, such as adsorption tank inlet and outlet pipelines, regeneration electric heater inlet pipelines, dryer inlet pipelines, purification tower inlet pipelines, subcooling separators, carbon dioxide spherical tanks, etc. When the system is overpressured, the safety valve starts to release pressure to meet the pressure relief requirements.

NEWTEK mainly expounds on the process route selection for the recovery and purification of high-concentration carbon dioxide in the tail gas emitted by the petroleum refining industry. According to the differences in carbon dioxide purity, impurity composition, process parameters, product quality requirements and recovery rate in the tail gas of different units, combined with the characteristics of various process methods for carbon dioxide recovery and purification, a suitable combined process method is selected to fundamentally solve the safety and product quality problems caused by changes in the impurity composition in the exhaust gas, so as to ensure the "safe, stable, full and excellent" operation of the unit.

Take the A₂O process as an example, through process principle analysis and field research of sewage treatment plants, combined with relevant specifications and standards, the conventional and indirect identification methods of the A₂O process operation status are determined in turn, and the identification basis based on terminal pollutant monitoring indicators, process parameters, sewage treatment facility power consumption, sludge discharge, equipment operation status, etc. is proposed. On this basis, the operation and maintenance skills in terms of process operation parameters, mud and water property indicators, equipment operation status, etc. are comprehensively analyzed, and a software that can quickly identify and analyze faults is designed, which provides a reference for realizing the intelligent, efficient and stable operation of the A₂O process.